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Atractus balzani BOULENGER, 1898

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Higher TaxaColubridae (Dipsadinae), Colubroidea, Caenophidia, Alethinophidia, Serpentes, Squamata (snakes) 
Common NamesBolivian Ground Snake 
SynonymAtractus balzani BOULENGER 1898: 129
Atractus balzani — PETERS et al. 1970: 27
Atractus balzani — WALLACH et al. 2014: 69 
DistributionNW Bolivia (La Paz)

Type locality: Missiones Mosetenes, NW Bolivia [15°47’S, 66°58’W, ca. 517 m elevation]  
TypesHolotype: MSNG 28873, a 400 mm specimen (L. Balzan, 1892). 
DiagnosisDescription: snout-vent length (SVL, hereafter) 354 mm; midbody diameter 8.4 mm (2.37% SVL); tail length 51 mm (14.4% SVL); head length 11.3 mm (3.19% SVL); head width 6.91 mm (61.1% head length). Head arched in lateral view; snout rounded in dorsal view; canthus rostralis poorly defined. Interocular distance 4.3 mm; snout-orbit distance 4.0 mm; rostral 1.2 mm high, 2.1 mm wide, visible in dorsal view; internasal 1.9 mm long, 0.9 mm wide, internasal suture sinistral with respect to prefrontal suture; prefrontal 3.2 mm long, 2.1 mm wide; supraocular subtrapezoidal, 1.2 mm long, 1.0 mm wide; frontal subpentagonal, 2.8 mm long, 3.1 mm wide; parietal 4.7 mm long, 2.9 mm wide; nasal divided, nostril restricted to anterior nasal; anterior nasal 0.65 mm long, 0.95 mm high; posterior nasal 1.1 mm long, 1.1 mm high; loreal long (2.3 mm long, 0.7 mm high); second and third supralabials contacting loreal; eye diameter 1.5 mm; pupil rounded; one small postocular (0.4 mm long, 1.2 mm high); temporals 1+2; anterior temporal 2.1 mm long, 1.1 mm high; anterior temporal in contact with parietal, postocular, and fourth–fifth supralabials; lower posterior temporal longer than upper posterior temporal; upper posterior temporal 2.0 mm long, 1.0 mm high; lower posterior temporal 2.4 mm long, 1.0 mm high; supralabials 6/6, third–fourth with orbit; second supralabial higher than first supralabial, and similar in size to third and fourth; fifth supralabial higher and sixth longer than remaining supralabials; symphysial damaged; infralabials 7 (right side) and damage in the right side, first four infralabials contacting chinshields; chinshields 1.9 mm long, 1.3 mm wide; ventrals 164; subcaudals 32; dorsal scale smooth in 14/17/16 rows; maxillary teeth 6/6. Dorsum of head dark brown; dorsal ground color of body reticulate dark brown, except for the first dorsal scale row mostly pale brown; dorsal scales with the center of each scale lighter (pale brown) and edges darker (dark brown); belly cream with few dispersed, small brown dots along body; dots concentrated on the lateral portion of ventral scales anteriorly to midbody; small brown dots on the posterior third of the body; tail creamish white heavily pigmented with dark brown spots on its ventral surface with yellowish cream background (Fig. 1A).

Passos et al. 2018 performed detailed comparisons between the holotype of A. balzani (Fig. 1; Table 1) and two species reported as occurring in the Yungas region of Bolivia, Atractus emmeli (Boettger 1888) and A. major Boulenger 1894 (Boulenger 1896: 645; Fig. 3). We can distinguish A. balzani from A. major by having an inconspicuous canthus rostralis, six supralabials, and the first four infralabials contacting chinshields (vs. very conspicuous canthus rostralis, seven supralabials, and first three infralabials contacting chinshields). By contrast, there is no feature in external morphology that unequivocally distinguishes A. balzani from A. emmeli, and Boulenger (1898) based his description of A. balzani mainly on its general color pattern, the presence of one postocular and 17 dorsal scales rows (supposedly counted on the midbody). Although inconsistent in the A. emmeli populations, all these features are polymorphic certainly in the Bolivian and Peruvian samples (Figs. 1–2; Table 1). In addition, the taxonomy of the Atractus emmeli complex was recently reassessed through an integrated approach (P. Passos et al. in prep.), with a considerable increase of the phenotypic variability range previously reported for the nominal species (e.g., McCoy 1971; Salazar-Bravo et al. 2010). Nonetheless, in the aforementioned studies, the authors were not able to examine the holotype of A. balzani, but now, with new data made available from its holotype, it is possible to infer the taxonomic status of the species. The only apparent difference distinguishing both species is the number of dorsal scale rows at midbody, being 15 in A. emmeli and 17 in A. balzani. However, Peters & Orejas-Miranda (1970: 25) reported one individual female of A. emmeli—from an unknown origin—with 17 dorsal scale rows, seven supralabials, two postoculars, 188 ventrals and 22 subcaudals. Additionally, we also corroborated the presence of polymorphic dorsal scale rows counts in A. emmeli based on two other specimens from Peru (MUSM 2313 provenance unknown and MUSM 27402 from Trompeteros, department of Loreto), which retained 17/17/17 and 15/17/17 dorsal scales, respectively. The A. balzani holotype is also polymorphic with respect to its dorsal scale rows, displaying 14/17/16. In addition, the differences in color pattern between the A. balzani holotype, the lectotype of A. emmeli (Fig. 1) and additional specimens of A. emmeli from Bolivia, being the pale occipital band, a dorsum which is uniformly black and belly which is almost fully cream are also polymorphic in the Bolivian populations of A. emmeli (Fig. 2). Thus, it is not possible to distinguish both species based on any of the traits mentioned above, and we conclude that the best taxonomic decision to be made at this time is to place A. balzani in the synonymy of A. emmeli. 
EtymologyNamed after Dr. Luigi Balzan (1865-1893), an Italian naturalist and collector of the holotype. 
  • Beolens, Bo; Michael Watkins, and Michael Grayson 2011. The Eponym Dictionary of Reptiles. Johns Hopkins University Press, Baltimore, USA - get paper here
  • Boulenger,G.A. 1898. A list of the reptiles and batrachians collected by the late Prof. L. Balzan in Boliva. Ann. Mus. Civ. Storia Nat. Genova (2) 19: 128-133 - get paper here
  • PASSOS, PAULO; VINÍCIUS SUDRÉ, GIULIANO DORIA & PATRICK D. CAMPBELL 2018. The taxonomic status of the “forgotten” Bolivian snakes, Atractus balzani Boulenger 1898 and Atractus maculatus (sensu Boulenger 1896) (Serpentes: Dipsadidae). Zootaxa 4438 (1): 176–182 - get paper here
  • Peters, James A.; Donoso-Barros, Roberto & Orejas-Miranda, Braulio 1970. Catalogue of the Neotropical Squamata: Part I Snakes. Part II Lizards and Amphisbaenians. Bull. US Natl. Mus. 297: 347 pp. - get paper here
  • Wallach, Van; Kenneth L. Williams , Jeff Boundy 2014. Snakes of the World: A Catalogue of Living and Extinct Species. [type catalogue] Taylor and Francis, CRC Press, 1237 pp.
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